The invention relates to capacitive sensors, in particular design approaches which reduce noise, in particular common mode noise.
Capacitive proximity sensors are based on detecting a disturbance in a capacitive coupling of sensor electrodes, either to ground or to another electrode, caused by the proximity of a pointing object, such as a human finger or a stylus.
Example devices include single control buttons, i.e. non-position sensitive detectors or so-called zero dimensional sensors, as well as position sensitive detectors which can be classified into one-dimensional and two-dimensional sensors.
One-dimensional capacitive sensors are found in linear or circular form as lighting controls, television remote controls, and in solid state MP3 portable music players, for example.
Two-dimensional capacitive sensors are found in touch screens, touch sensitive keyboards and key pads, for example, and are commonly used in consumer electronic devices and domestic appliances. Such sensors are often used in conjunction with an underlying display, such as a liquid crystal display (LCD), or cathode ray tube (CRT).
Other devices which may incorporate position sensitive capacitive sensors include pen-input tablets and encoders used in machinery for feedback control purposes, for example. Position sensitive capacitive touch sensors are capable of reporting at least a 2-dimensional coordinate, Cartesian or otherwise, related to the location of an object or human body part, by means of a capacitance sensing mechanism.
Devices employing position sensitive capacitive touch sensors have become increasingly popular and common, not only in conjunction with personal computers, but also in all manner of other appliances such as land line and mobile telephones, personal digital assistants (PDAs), point of sale (POS) terminals, electronic information and ticketing kiosks, kitchen appliances and the like.
Capacitive touch sensors can be classified into two types in terms of the manner in which the signal is sensed, namely single-ended electrodes and adjacent transverse electrodes.
U.S. Pat. Nos. 5,730,165 and 6,466,036 describe a capacitive sensing device of the single-ended electrode type which relies on measuring the capacitance of a sensing electrode to a system reference potential, most commonly earth or ground. The pointing device, typically a human finger or a stylus, thus effectively defines ground and may be thought of as the second “plate” in the capacitor, with the sensing electrode forming the first “plate”. The user's actions may then be considered as varying the separation between the plates of a capacitor, and hence vary the measured capacitance.
U.S. Pat. No. 6,452,514 describes a capacitive sensing device of the transverse electrode type which is based on measuring capacitive coupling between two adjacent electrodes. In such a sensor, one electrode, the so-called drive electrode, is supplied with an oscillating drive signal, and this signal is capacitively coupled to another electrode, the so-called sense electrode. The degree of capacitive coupling of the drive signal to the sense electrode is determined by measuring the amount of charge transferred to the sense electrode by the oscillating drive signal. The amount of charge transferred, i.e. the strength of the signal seen at the sense electrode, is a measure of the capacitive coupling between the electrodes, which is influenced by the proximity of the pointing object. The pointing object therefore may be considered to change the dielectric environment, and thus the field, between the two “plates” of the capacitor formed by the drive and sense electrodes, and hence change the measured capacitance.
It is known in the art that capacitive touch (or proximity) sensor of the transverse electrode type are prone to noise problems, in particular common mode noise injected into the raw signal collected during sensing, and the invention aims to solve or mitigate this problem.